Abstract
Purpose :
During photoreceptor degeneration monocytes from the periphery can infiltrate the retina and join the resident microglia in the immune response to neuronal loss. After photoreceptor loss is complete ramified macrophages re-tile the retina, but this population remains heterogeneous. Here we compared subtypes of macrophages before and after photoreceptor degeneration.
Methods :
Using a light-inducible model of photoreceptor degeneration (Arr1-/- mice), we investigated retinal macrophages before and after photoreceptor degeneration (0 and 20 days of light exposure, respectively). To compare subtypes of macrophages in the retina, we used flow cytometry to quantify cell numbers, immunohistochemistry to quantitatively examine cellular morphologies, and single-cell mRNA sequencing (scRNAseq) to investigate gene expression. All mice were handled according to ARVO, NIH, and UC Davis IACUC guidelines.
Results :
After photoreceptor degeneration is complete, the retina contains a heterogeneous population of macrophages. ScRNAseq revealed transcriptionally distinct subpopulations, including one population that highly expressed genes associated with a monocytic lineage in addition to genes similarly expressed by mildly activated microglia, such as Aif1, Lyz2, and Sepp1. Using a fluorescent lineage tracing paradigm to conclusively identify microglia and monocyte-derived macrophages (Arr1-/- Ai9KI/KI Cx3cr1+/YFP-CreER), we find that monocyte-derived macrophages comprise approximately half of all retinal macrophages after photoreceptor degeneration is complete. Additionally, these monocyte-derived macrophages reside alongside bone fide microglia throughout the retina, although they are typically less morphologically complex than the resident microglia.
Conclusions :
Following photoreceptor degeneration, a heterogeneous population of macrophages remain in the retina. This population consists of both microglia and monocyte-derived macrophages. The monocyte-derived macrophages adopt a range of microglia-like characteristics, including expression of genes such as Aif1, spatial tiling throughout the retina, and a ramified morphology. However, differences in transcriptional signatures, morphological complexity, and response to further injury suggest these cells with disparate etiologies may remain functionally distinct populations over prolonged periods of time.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.